RRULE export

[tlvenn]

It would be super convenient to be able to have a way to export rules in the text format compatible with RRULE such as:

"FREQ=WEEKLY;DTSTART=20120201T093000Z;INTERVAL=5;UNTIL=20130130T230000Z;BYDAY=MO,FR"

[pbogut]

I can see how that could be usfull. I guess we could add to_string and maybe from_string. Fancy to create PR?